1. Field of the Invention
The present invention relates to a noise controller for performing an active noise control to suppress an unwanted ambient noise.
2. Description of the Invention
A conventional noise controller for such an active noise control is disclosed in International Patent Publication WO88/02912, in which a control signal is produced by feeding an adaptive filter with data of e.g. the rotational speed of an engine which is a possible source of noise. The control signal transmitted to a control speaker for generating a control sound to cancel the noise. Also, a difference between the control sound and the unwanted noise is measured by an error detector and the coefficient of the adaptive filter is updated so that the difference becomes minimum in level. However, the arrangement suggested in WO88/02912 permits the detection of only a periodic component of the noise from the engine and fails to detect a random component, e.g. a road noise, the same which then remains unsuppressed.
One more noise detector for detection of the random component may be added to the conventional noise controller. However, if the periodic component is greater than the random component, its change in the gain and phase will vary the filter response of the adaptive filter which in turn causes the random component to remain intact. When the random component is high in proportion, the adaptive filter becomes subjected to a noise transfer function and will not be responsive to suppress the periodic component which varies in both amplitude and phase. Also, the adaptive filter fails to stay uniform in the filter response when the ratio in level between the periodic component and the random component is varied with time.